As a method replacing a conventional radiography, a radiation image storing and reproducing method utilizing a stimulable phosphor was proposed, and is practically employed. The radiation image storing and reproducing method employs a radiation image storage panel (i.e., stimulable phosphor sheet) comprising a stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the storage panel to absorb radiation energy having passed through a subject or having radiated from a subject so as to excite the stimulable phosphor; sequentially re-exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (i.e., stimulating light) to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain a series of electric signals; and reproducing the radiation image of the object as a visible image from the series of electric signals. The storage panel thus treated is subjected to a step for erasing a radiation energy remaining therein, and then stored for the next image storing and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
The radiation image storage panel employed in the above-mentioned procedure has a basic structure comprising a support and a stimulable phosphor layer provided thereon. If the phosphor layer is self-supporting, the support may be omitted. On the free surface (surface not facing the support) of the phosphor layer, a transparent protective film is generally placed to keep the phosphor layer from chemical deterioration or physical damage.
The phosphor layer generally comprises a binder and stimalable phosphor particles dispersed therein. However, it may consist of agglomerated phosphor with no binder. The phosphor layer containing no binder can be produced by deposition process or firing process. Further, the phosphor layer comprising agglomerated phosphor soaked with a polymer is also known. In any types of phosphor layers, the stimulable phosphor releases a stimulated emission when re-excited with a stimulating light after having been exposed to a radiation such as X-rays. Accordingly, the radiation in the form of an image having passed through a subject or radiated from a subject is absorbed by the phosphor layer of the storage panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the storage panel in the form of a latent radiation energy-stored image. The latent radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating light. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
Even in the radiation image storing and reproducing method described above, it is naturally desired that a radiation image is reproduced with a high sensitivity and with good image quality (such as a high sharpness and a good graininess). Further, it is desired to make the apparatus for the radiation image storing and reproducing method as compact as possible with a lower cost. It is also desired that the radiation image is reproduced as quick as possible.
EP 1 113 292 A2 discloses a new radiation image reproducing system employing a combination of a radiation image storage panel containing a stimulable phosphor (i.e., energy-storing phosphor) and a fluorescent sheet which absorbs a radiation and subsequently emits a light of a ultraviolet to visible region (i.e., radiation-absorbing phosphor). Thus, in the above-mentioned new system, the function of absorbing a radiation and the function of storing the absorbed radiation are separated from each other, and each function is allotted to a separate element. In this system, the radiation having passed through a subject or other imagewise radiation is first converted in the element containing the radiation-absorbing phosphor to give a light emission of a ultraviolet to visible region (i.e., first stimulation); the light is then absorbed and stored in the energy-storing phosphor in the form of a latent energy; the stored energy is then released as a light emission by re-exciting the phosphor with a stimulating light (i.e., second stimulation); and the last light emission is photoelectrically read to obtain a series of electric signals for reproducing the radiation image.
It is known that certain stimulable phosphors such as ZnS:Cu can be excited by application of a light of a ultra-violet light to visible region to store the energy and then stimulated by electric field to give a light emission. This phenomenon is called “Gudden-Pohl” effect.
Japanese Patent Provisional Publication No. 62-69182 describes a two-dimensional radiation detecting device utilizing the Gudden-Pohl effect. The disclosed detecting device comprises a stimulable phosphor layer, an electrode layer or a group of electrode layer which stimulates the stimulable phosphor with electric field, and a two-dimensional detecting layer. These layers are combined adjacently with each other to form a multi-layered composite structure. On the composite structure, a radiation is impinged to form in the stimulable phosphore layer a latent image of the radiation. Thereafter, the stimulable phosphor layer is stimulated by electric field to give a light emission which is instantly detected in the two-dimensional detecting layer as a photoelectric current.
U.S. Pat. No. 4,818,877 describes a memory display system which includes a recording support medium having a layer of a luminescent material capable of storing energy coming from a light beam such as an X-ray beam, electrodes enclosing the luminescent material and applying thereto an electric field for releasing the previously stored energy in the form of a beam; an optical transmission device placed in the path of said beam; a sensor receiving this beam and converting the intensity of the light beam into an electric signal; and a processing circuit receiving said electric signal, processing it and controlling display thereof on a display device.
In each radiation image-reproducing system, the radiation energy is absorbed and stored only in the phosphor layer to be stimulated with the electric field. Therefore, it is difficult to realize both of a high radiation-absorbing efficiency and use it with a high electric field in each phosphor layer.